Internals: Windows hidden with HiddenFramesRegular (but NOT HiddenFramesForResize) preserve their SizeContents, so restoring a auto-resize window after it's been hidden by tabs won't reset its size for a frame. Arguable. Let's see how it goes. (Followup to b48e295bddbf965d7382ec5578ed05d2fe601114)

docs/CHANGELOG.txt

@@ -51,6 +51,7 @@ Other Changes:
 - Added ImGuiWindowFlags_UnsavedDocument window flag to append '*' to title without altering 
   the ID, as a convenience to avoid using the ### operator.
 - Window: Contents size is preserved while a window collapsed. Fix auto-resizing window losing their size for one frame when uncollapsed.
+- Window: Contents size is preserved while a window contents is hidden (unless it is hidden for resizing purpose).
 - Window: Resizing windows from edge is now enabled by default (io.ConfigWindowsResizeFromEdges=true). Note that 
   it only works _if_ the back-end sets ImGuiBackendFlags_HasMouseCursors, which the standard back-end do.
 - Window: Added io.ConfigWindowsMoveFromTitleBarOnly option. Still is ignored by window with no title bars (often popups).

imgui.cpp

@@ -4334,6 +4334,8 @@ static ImVec2 CalcSizeContents(ImGuiWindow* window)
 {
     if (window->Collapsed)
         return window->SizeContents;
+    if (window->Hidden && window->HiddenFramesForResize == 0 && window->HiddenFramesRegular > 0)
+        return window->SizeContents;
 
     ImVec2 sz;
     sz.x = (float)(int)((window->SizeContentsExplicit.x != 0.0f) ? window->SizeContentsExplicit.x : (window->DC.CursorMaxPos.x - window->Pos.x + window->Scroll.x));